Even I consider this to be gamey... and the interface will allow it...
Offering tech you
do not possess in a trade!
The game does not verify what you offer, you can offer anything, and receive a bunch of tech for nothing. It's the honor system.
Most times when this happens, this can be viewed as a breach of honor, and that player will not be liked very much.
Gray areas do apply, like offering a gift for expectation of a gift in return... but before the return gift can be offered, your empire relations go sour, and you go to war, which kind of ruins the whole gift exchanging thing.